Retractable assemblies are widely used in analytical measurements technology. They serve, without process interruption, to withdraw sensors from the process, even in the case of high process pressures, and then to reintroduce the sensors back into the process. For this, the sensor is automatically or manually moved back and forth between a measuring position and a maintenance, or service, position. In the maintenance position, the sensor is checked, calibrated, replaced or just simply cleaned, which occurs in a wash/rinse/calibration chamber, and depending on the application, is of great importance for the quality of the measurements. The process variable to be determined or monitored is ascertained in the measuring position. Used as a drive unit is usually a pneumatic cylinder, which is arranged concentrically around the tubular holder or the immersion tube. The drive unit and wash/rinse/calibration chamber are, in most cases, so arranged that process medium can reach the drive unit in the case of malfunction or leakage.
Two different solutions are used for preventing process medium from getting into the retractable assembly: In a first form of embodiment, a ball valve, or a plug valve is used, which via a rotating, e.g. spherical element opens or closes the opening through which the sensor is moved into and out from the process. In a second form of embodiment, a closing plug is used. This closing plug is an integral component of the retractable assembly.
Ball valve assemblies are preferably applied in media with a solids fraction. Solids are understood to be fibers, as well as clinging, baked-on lime and similar materials. In ball valve assemblies, the sensor is conveyed into and out of the process out in isolation from the closing/opening mechanism. For this, the ball valve is closed or opened. For the purpose of cleaning the sensor, the sensor is brought into the maintenance position in a rinsing chamber separated from the process.
Retractable assemblies are available and sold by the assignee in different embodiments under the name ‘CleanFit’. For example, an assembly with a closing plug is CleanFit S, CPA 471-474. In these retractable assemblies, the holder for the sensor is itself embodied as a sealing element. The front part of the holder is embodied as a plug, which already radially seals off the process during the retraction. The construction of the sealing system of the assemblies CPA 471 and CPA 472 assures, in such a case, a perfect isolation between the rinsing chamber, and, therefore, the ‘environment’, and the process, and, indeed, in every position of the sensor holder.
Relatively complex open loop control, or closed loop control solutions are used for the purpose of detecting the correct assuming of the measuring or service position. In order to monitor the correct positioning of the tubular holder or immersion tube, end position switches are usually utilized. In general it can be said that only two positions are implementable with the known solutions without an overly large technical effort. The assuming of a third position, in which comprehensive cleaning and washing or rinsing concepts of all components contacting the medium can be taken into consideration, for example, leads to a technically complex solution.
The known solutions are not quite problem free in that there is no reliable spatial isolation of the wash/rinse/calibration chamber, the immersion tube and the assembly drive. As a result, in the case of an unnoticed leak in the region of the rinsing chamber, process medium can reach the drive unit, which can lead to a failure of the drive unit; moreover, it is possible that the process medium can reach the environment via the drive unit.
In order to assure that the sensor is always arranged in a defined position in relation to the immersion tube and therewith the process, a separate twist preventer is provided in the known solutions. This is usually externally built on and causes an increased functional risk as well as extra costs.